Go-No Go Mercury 3.5 Help please

[shantycrew]

My mercury 3.5 outboard has developed a Go-No go problem that has got me stumped. Not only is it difficult to start, (used to go 1st or second pull) but when it fires up it's revs fluctuate dramatically, and only goes at all with the accelerator on full. As soon as I throttle back the engine dies, and is a pig to start again. I've eliminated the possibility of it being the carb by swopping it for one from another 3.5 which was working fine and the same problem occurs with that on my engine.
The up and down revs are not erratic but a constant rhythm, it will drive the boat but in fits and starts.
Any idea what this could be.

[a64pilot]

Vacuum leak is my guess


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[LakeSuperior]

Symptoms are also indicative of plugged low speed jet and plugged metering (?) holes in the high speed jet. This is now a common occurrence because of the ethanol in the fuel.

[a64pilot]

Originally Posted by LakeSuperior

Symptoms are also indicative of plugged low speed jet and plugged metering (?) holes in the high speed jet. This is now a common occurrence because of the ethanol in the fuel.

Except, he changed carbs, put a know good carb on, so probably not the carb.
But his symptoms do sound like a lean mixture, either from a gummed up carb, or un-metered air, a vacuum leak.
It could be ignition too, but a vacuum leak is usually easy to check for.


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[LakeSuperior]

Originally Posted by a64pilot

Except, he changed carbs, put a know good carb on, so probably not the carb.


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Thanks, I need to work harder on my reading comprehension.

[belizesailor]

Originally Posted by a64pilot

Vacuum leak is my guess


...

That or a fuel pump issue. Dont know this specific motor, does it have a diaphram pump like many OB's? A pin hole in the diaphram or leaky pump gasket can cause this.

Or, a dirty fuel filter.

[a64pilot]

It runs at full throttle, an engine with a vacuum leak will run at full throttle because there is essentially almost no vacuum in the intake, because the throttle is wide open.
If the pump was faulty or the fuel filter clogged, both would limit fuel flow, and then the engine would run well until it was under load.

Wild card is ignition, anything is likely to happen with ignition messed up.


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[Trim50]

If it is a four stroke, the best known fix is to attach a chain to it and drop it off the bow of the boat as an anchor.

[Scaramanga F25]

Water in your gas?

[shantycrew]

Thanks crew, Now how do I check for a vacuum leak, and where do I check it and what with. I'm wondering if the actual fuel is to blame as It's sat on deck in a jerry can for over six months and was bought from an auto garage in Luperon in the Dominican Republic. There's some stuff I've read where there's some sort of degredation that occurs, anyone know about this. Thing is that when I do get the engine to start there is no response from the throttle only full works then It goes from full to almost idle and back again cycling like that every few seconds, like a pulse action. In fact yesterday it pushed the boat for about 200 yds acting normal before the same thing started to occur, just about got me back to my boat before it died on me. Oh BTW the engine is only eighteen months old, but I'm no-where near a mercury dealer or I'd have them look at it under warranty.

[Scaramanga F25]

After I had bad gas i.e. water in my tank I had to have the carb removed and cleaned out.
Been there done that twice. My daughter assured me new gas was in the tank. NOT!

[sparrowhawk1]

Are you using a separate fuel tank or a tank on top of the engine

[shantycrew]

The tank on the top of the engine, although the screw top threaded plastic bit snapped and now just have a rubber bung in there, with a breath hole in it of course.

[capt jgw]

Fresh gas? My 3.5 Nissan (same engine if yours is a two stroke) runs like crap on old gas. Fill it up with fresh and it runs great. I've taken to dumping the old fuel into my snow blower for the winter. If I leave it in the outboard it won't run in the spring. The snow blower doesn't seem to care. (I know, a lot of you are going to say "what's a snow blower? I'm working on being able to forget what it is.)

[shantycrew]

Thanks Cpt. That's going to be my next move then, old gas out, fresh new shiny gas in. watch this space. Oh and you wouldn't need that snow blower down here in Puerto Rico, just a blower will do